Blue-phase liquid crystals (BPLC) are locally-isotropic fluids in which the liquid crystal molecules organize themselves into complex three-dimensional (3D) structures that are characterized by crystallographic space group symmetry, whereby the blue-phase liquid crystals form double-twisted cylinders that are separated by defect lines. Specifically, as temperature increases, blue-phase liquid crystals enter one of these blue-phase (BP) network states, which are identified as: BP I, II and III. The blue-phase liquid crystals in the BP I and BP II network states, as shown in FIGS. 1A and 1B respectively, form soft, frequently coagulating platelet-domains, which are micrometer to sub-millimeter in size. Blue-phase liquid crystals in the BP I network state have a Bravis lattice that is body-centered, while the liquid crystals in the BP II network state have a Bravis lattice that is a simple cubic. However, blue-phase liquid crystals in the BP III network state have a cloudy and amorphous appearance, which is referred to as “blue fog”, whereby light is selectively reflected, with light-scattering vectors forming a reciprocal Bravis lattice of a cubic periodic system.
As such, blue-phase liquid crystal (BPLC) materials have the potential to serve as a next-generation liquid crystal display (LCD) material due to their desirable operating features, which include field-induced birefringence, fast response or switching time between light-scattering and light-transmitting states, which may be in the sub-millisecond range, and that is at least one order of magnitude faster than that attained by current nematic liquid crystal (NLC) type displays. Blue-phase liquid crystals are also desirable materials for LCD displays, as blue-phase liquid-crystal based devices and materials do not require a surface-alignment layer, which is normally required in standard LCD displays. As a result, the fabrication process of blue-phase liquid-crystal based LCDs and other devices is greatly simplified, and as a result, time and manufacturing costs are reduced.
Polymer-dispersed liquid crystals (PDLCs) are a class of optical materials that can be prepared by polymerization or through solvent evaporation-induced phase separation. PDLCs typically include micron-sized liquid crystal droplets that are encapsulated in matrices of optically transparent polymers. Specifically, the liquid crystal molecules nucleate and form droplets with a disparity in size and shape that depends on the particular attributes of the phase-separation process used, such as the rate of polymer gelation, for example. Thus, at zero-applied voltage, the indices of refraction between the polymer and the liquid crystal molecules are mismatched, causing the phase-separated PDLC film to normally appear milky and scatter incident ambient light. As a result, PDLC-based films can be switched from a light-scattering state to a light-transparent state or vice versa in response to an applied voltage. Compared to conventional nematic liquid crystal (NLC) type displays and devices, PDLC devices have many advantages, including high light transmittance and the lack for the need of polarizers and alignment films. Additional advantages of blue phase liquid crystals (BPLCs) include field-induced birefringence due to their sub-millisecond response time, which is at least one order of magnitude faster than the present nematic liquid crystal (NLC) based displays. Another significant advantage of BPLCs is their wide and symmetric viewing angle due to the fact that their “voltage off” state is optically isotropic and the “voltage on” state forms multidomain structures. Consequently, PDLC devices have been used in broad applications, ranging from switchable light modulators and smart windows to information displays, switchable lenses and holographically-formed optical elements and devices, for example.
Thus, it would be desirable to incorporate blue-phase liquid crystals (BPLC) into a polymer-dispersed liquid crystal (PDLC) material to form a polymer-dispersed blue-phase (PDBP) liquid crystal material, such as a film, which incorporates the benefits of typical PDLC materials (high light transmittance, lack of need of polarizers and alignment films) with that of blue-phase (BP) liquid crystals (field-induced birefringence, fast-response/switching time between light-scattering and light-transmitting states at low voltage levels). However, manufacturing such PDBP materials is made difficult in part due to the inability of the blue-phase liquid crystals to achieve their blue phase at room temperature. That is, blue phases, (i.e. the self-organized three-dimensional structures formed by double-twisted cylinders of cholesteric liquid crystals LCs), appear only in a narrow temperature range between the chiral nematic (cholesteric) and isotropic phases. Thus, the inherent narrow blue phase temperature range is one of the most significant limitations restricting the potential applications of blue phase liquid crystals. One of the methods used to enlarge the blue phase temperature range is the stabilization of the defects by polymerizing a small amount of reactive monomers in the defect regions approximately around 3 volume % to 5 volume % of the cubic lattice, wherein polymerized reactive monomer molecules forms polymer network within the disclination core and stabilize the appearance of liquid crystal blue phase for wide temperature range. There is no encapsulation of the blue phase liquid crystal and no continuous phase of the polymer.
Therefore, there is a need for a polymer-dispersed blue-phase (PDBP) liquid crystal material, such as a film that achieves its blue phase at room temperature to facilitate its fabrication and its use in various devices, such as optical retardation films, switchable light shutters, LCD display devices, and the like. Furthermore, there is a need for a polymer-dispersed blue-phase (PDBP) liquid crystal material, such as a film that is compatible for use in an electro-optical cell, such as an IPS (in-plane switching) cell, that utilizes flexible or drapeable substrates that may be mechanically flexed, bent, or deformed. Moreover, there is a need for a polymer-dispersed blue-phase (PDBP) liquid crystal material, such as a film, that has high light transmittance, lacks the need for use of polarizers and alignment films, allows field-induced birefringence, and provides fast-response/switching times between light-scattering and light-transmitting states. In addition, there is a need for a polymer-dispersed blue-phase (PDBP) liquid crystal material, such as a film, that is compatible for use with a continuous fabrication processes used to manufacture optical devices, such as LCD displays.